Soldering is an effective method for joining metallic components and can even join many non-metallic components provided that the faying surfaces are suitably metallized. Accordingly, many types of solders, soldering processes, and solder joint designs are known. Of the many types of solder joints, one of the most difficult to make is a large-area joint that provides a hermetic seal and is free of internal voids. In this context, a large-area solder joint is a solder joint covering an area having a smallest dimension more than 2 mm long.
Hermetic seals are difficult to form with large-area joints because the most reliable method of guaranteeing joint hermeticity is to ensure the formation of a continuous edge fillet around the entire perimeter of the joint. Meeting this requirement becomes progressively more difficult for joints having larger areas and perimeters.
Voids are a problem for a large-area solder joint because at dimensions greater than 2 mm, gas bubbles that are trapped between components or evolve internally on heating to the soldering temperature cannot overcome the hydrostatic pressure of the molten solder and escape via the edges of the joint. The bubbles thus remain trapped in the solder and form voids when the solder solidifies. FIG. 1 illustrates the dependence of the percentage of voids in a solder joint on the minimum joint dimension for some conventional solders. As can be readily seen from FIG. 1, the problem of voids in solder joints increases with the dimensions of the joints. These voids generally impair the electrical, thermal, and mechanical properties of a solder joint.
Making a large-area joint without flux further increases the difficulty of making void free joints. In general, a flux helps to remove surface oxides and thereby promotes wetting and spreading of molten solder. Without flux, making good quality solder joints is inherently more difficult, but avoiding the need for flux can simplify a soldering process. Accordingly, fluxless processes and technologies have been devised for making solder joints that are thin, large-area, and void-free. Some of these techniques include pre-applying solder, the xe2x80x9cpressure variationxe2x80x9d process, and applying compressive stress during the thermal cycle of the soldering. For best results, all three methods can be combined.
Pre-applying solder applies solder to the surface of one or both of the components being soldered, thereby decreasing the number of surfaces in the joint and hence sources of voids.
The pressure variation process reduces void levels in solder joints by compressing the trapped gas bubbles so that the gas bubbles and resulting voids occupy a much smaller fraction of the joint volume. The pressure variation process generally uses external gas pressure in a way that has many analogies with hot isostatic pressing. A typical pressure variation process involves placing the assembly of components to be soldered in a chamber at reduced pressure (P1) and heating the assembly to the peak process temperature. The pressure in the enclosure is then increased several orders of magnitude to a higher pressure (P2), and the assembly is allowed to cool under the high pressure P2. To the extent that the bubbles behave as an ideal gas, an initial volume V1 of voids at pressure P1 decreases to a volume V2 of voids at pressure P2, where volumes V1 and V2 are related as indicated in Equation 1.      Equation    ⁢          xe2x80x83        ⁢    1    ⁢          :                  V      2        =                  V        1            ·              (                              P            1                                P            2                          )            
Equation 1 illustrates that the greater pressure P2 is in relation to pressure P1 the more effective the process is at reducing voids. Practical work has shown that a pressure ratio of 10:1 can typically achieve a void level of about 15%, and a ratio of 30:1 can reduce void levels to as low as 5%.
Difficulties arise with the pressure variation method when the solder joint is required to form a hermetic seal around a closed cavity. If the molten solder seals a closed cavity by wetting all of the joint surfaces, any variation in internal or external gas pressure can blow the solder off the joint line, thereby breaking the seal. Thus, the pressure variation process cannot be used with parts including solder seals around closed cavities.
In view of the limitations of known soldering techniques and solder joints, soldering processes and joints are sought that are able to provide thin, large area joining that is essentially void free and capable of hermetically sealing a cavity.
In accordance with an aspect of the invention, a vented cavity is formed between surfaces of components being joined with a large-area solder joint. The cavity reduces the distance that gas bubbles in molten solder must travel to escape during formation of the large area solder joint. Accordingly, fewer gas bubbles are trapped, resulting in fewer voids in the solder joint. Additionally, since the cavity is vented, a pressure variation process can be applied during soldering to improve the fill and hermeticity of the solder joint. The vent can be sealed after forming the solder joint to hermetically seal the cavity, if desired.
The vented cavity with or without the pressure variation process can be applied not only to solder joints but also to joints formed using a braze or an adhesive.
One embodiment of the invention is a process for attaching components. The process begins by forming an assembly including a first component and a second component with a joining material such as a solder, a braze, or an adhesive sandwiched between the first and second components. The first and second components form a vented cavity that the joining material surrounds. Heating the assembly activates or melts the joining material and gas bubbles in the joining material during heating can escape from the joining material via the cavity and the vent to the surroundings of the assembly. Sealing the vent after the joining material solidifies can hermetically seal the cavity.
After heating of the assembly, pressure surrounding the assembly can be increased to compress gas bubbles that may still remain trapped in the joining material. The increased pressure is maintained while cooling the assembly to solidify the joining material, so that any voids corresponding to the gas bubbles are smaller than they would be in a process that did not increase the pressure. Since the cavity is vented, pressure inside the cavity is same as the pressure outside the assembly and the increased pressure does not disturb hermeticity of the seal.
Another embodiment of the invention is a joined structure including first and second components made of materials such as a metal (e.g., molybdenum), a semiconductor (e.g., silicon), a glass, or a ceramic with a joining material such as a solder, a braze, or an adhesive sandwiched between the first and second components. The first and second components form a cavity that the joining material surrounds, and a vent leads away from the cavity. The vent can be sealed after the first and second components are joined so that the joining material and the vent together hermetically seal the cavity. The joint structure can further include a series components and solder joints forming a series of vented cavities that share a common vent, and/or a set of components that have individually vented cavities.